Palaeoenvironmental reconstruction of lacustrine source rocks in the lower 1st Member of the Shahejie Formation in the Raoyang Sag and the Baxian Sag, Bohai Bay Basin, eastern China

The lower 1st Member (Es1L) of the Oligocene Shahejie Formation is the most important source rock interval in the Raoyang Sag and the Baxian Sag. To reveal the controlling factors that affected the distribution of organic rich source rocks, we analyzed the lower Esl (Es1L) samples collected from the Raoyang Sag and the Baxian Sag using sedimentological and geochemical techniques to reconstruct the lacustrine palaeoenvironmental conditions. Most of the samples had chemical index of alteration (CIA*) values between 70 and 75, which reflect a stable and moderately weathered source under a transitional climate. The distribution of depocenters and palaeo-water depths indicated that the main catchment area was concentrated in the Raoyang Sag, with a maximum palaeo-water depth of 14 m and a wave-base depth of 7.5 m. Stable isotope data from carbonates (delta O-18(carb) and delta C-13(carb)) showed that the lakes with long residence times in the Raoyang Sag had a positive correlation and that the lakes with short residence times in the Baxian Sag had no isotopic covariance during the deposition of the lower Esl. The P/AI and P/Ti ratios, P-total and carbon isotope values of the organic matter (delta C-13(org)) in the two sags illustrated stable palaeoproductivity without much fluctuation. The enrichments of Mo, U and ratios of (C:P)(org), B/Ga and Sr/Ba further indicated saline and anoxic bottom water conditions associated with stable water-column stratification in Well Xi63 and brackish and less anoxic bottom water conditions in Well ND1 during the deposition of the main source rock of the lower Esl. Moreover, the plots of Mo and U enrichment versus TOC and hydrogen index values versus Delta delta C-13 (i.e., the difference between the delta C-13 values of carbonates and organic matter) corroborated the inference that the preservation of organic matter was the main factor controlling the formation of organic-rich source rocks in the Raoyang Sag and the Baxian Sag during the deposition of the lower Esl.